Category HIGHWAY ENGINEERING HANDBOOK

Multiple sign supports are designed to operate correctly when either one or all of the sup­ports within a 7-ft (2100-mm) radius are impacted. When only one support is impacted, the remaining signpost should support the sign and prevent it from penetrating the wind­shield. The desired impact performance of slip base and frangible coupler designs for large sign supports is depicted in Fig. 7.29. The base releases upon impact and the impacted support rotates up, allowing the vehicle to pass underneath the sign. This requires that the post be cut, at least 7 ft (2100 mm) above the ground, to provide a hinge for rotation.

As shown in Fig. 7.30 and discussed in Art. 7.5.3, hinges for large sign supports consist of three basic designs: (1) partially cut post with front friction plate, (2) completely cut post with front friction and rear hinge plate, and (3) completely cut post with weakened front plate and rear hinge plate. Proper performance of the hinge requires the correct selection of plate size, bolt size, and torque. Figure 7.51 and Table 7.16 present the design values for friction plates. Figure 7.52 and Table 7.17 present the design values for hinge plates. The bolt torque values for both friction and hinge plates are the same as presented as Table 7.15 for slip bases. Proper sized flat washers should be used under each nut and the head of each bolt.

The hinge systems shown in Fig. 7.30 are all unidirectional designs and should not be used in areas requiring bidirectional breakaway performance. Only the Transpo hinge system shown in Fig. 7.31 offers bidirectional breakaway capability.

Slip base designs for multiple sign supports are usually of horizontal design as shown in Figs. 7.23 and 7.26. Horizontal slip bases, when used in multiple-sign-support systems, operate satisfactorily when impacted from only one direction. Horizontal slip bases should not, therefore, be used for multiple sign supports where there is a high probability of impacts from more than one direction. In Fig. 7.26, the keeper plate prevents the bolts from “walking” out of the assembly as a result of wind vibration (Art. 7.4). The washers should separate the upper and lower slip plates by at least И in (3 mm), but not more than И in (6 mm), to prevent mating of the surfaces and possible binding due to friction. Proper size washers must also be used under the nut and bolt head to prevent the washers from deforming into the slots of the slip plates and binding the mechanism.

A typical concrete foundation detail is shown in Fig. 7.50, and specifications for the anchor piece of slip base designs are presented in Table 7.14. Notice that the foun­dation design includes eight reinforcing bars spaced around the anchor piece. This is a typical installation which is effective in maintaining the integrity of the foundation

a BARS V

DRILLED SHAFT (CONCRETE)

FIGURE 7.50 Horizontal base and concrete foundation detail.

under vibrations resulting from environmental loads. State specifications should be consulted to determine if local requirements deviate from details shown in Fig. 7.50.

Proper functioning of the slip base requires correct selection of bolt size and torque. Table 7.15 gives typical design specifications for large roadside sign slip bases and concrete foundations.

An omnidirectional triangular base, mounted in standard soil with three 2V2-in X 21/2-in (64-mm X 64-mm) perforated steel tube supports located within a 7-ft (2100-mm) radius, has been approved for use [49].

Transpo Industries manufactures a series of breakaway systems for ground-mounted sign supports, marketed under the trade name Break-Safe. The Break-Safe system uses frangible couplers and is available for U-channel, both concrete footing and direct burial; 3-in to 4.5-in (75-mm to 115-mm) round pipe; 3-in to 5-in (75-mm to 127-mm) square

tube; and wide-flange and standard beam shapes. Schematics of Transpo Industries concrete base and the direct burial system for back-to-back U-channel are presented in Fig. 7.47. Figure 7.48 shows the Break-Safe system for square-tube and round-pipe supports [33].

The Break-Safe has a number of advantages over slip base designs. One advantage is that the critical torque requirements of the slip base bolts are eliminated by the use

of frangible couplings. There are retrofit kits available, for wide-flange and standard beam supports, that use the existing slip base anchor to convert to a frangible coupler design. Another advantage of the Break-Safe system is that the concrete base installa­tions do not require a protruding stub. This decreases the probability of snagging the undercarriage of an impacting vehicle, and damage to the anchor system itself. The protruding stub is eliminated by bolting the frangible coupling into anchors placed in the concrete footing. Proper assembly requires that the anchors be accurately placed for each type of support system. Accurate anchor placement requires the use of an installation jig similar to Fig. 7.49 [33]. The anchor spacings, dimensions A and B in Fig. 7.49b, vary with the type and size of sign support being installed. Frangible and load concentration couplers (Fig. 7.22) usually perform satisfactorily when struck from any direction (they are multidirectional).

Multimount sign supports have two or more support posts of breakaway design. The breakaway mechanism is either a fracture or a slip base type. Fracture mechanisms consist of frangible couplers or frangible one-piece posts.

7.8.1 One-Piece Multiple-Mount Sign Supports

Direct burial assemblies that are approved for use include dual 3-lb/ft (4.5-kg/m) U-channel and dual 4-in X 4-in (90-mm X 90-mm) wooden posts that have been modified with two 1.5-in (38-mm) holes placed at 4 and 18 in (100 and 450 mm) above the ground line. Other than these exceptions, multiple-mount sign supports require the use of anchor pieces, sleeves, slip bases, or frangible couplings for acceptable impact performance. Only those devices approved for use by the FHWA should be used for multiple-mount supports. Since more than one support can be simultaneously struck by an errant vehicle, it is important that proper installation procedures be followed. Figure 7.46 illustrates the holes drilled in a direct burial wooden post to provide the weakened cross-section required for multiple wooden post installation.

The following guidelines should be followed for fastening sign panels to supports:

• Bolts smaller than %s in (8 mm) should not be used to fasten sign panels to the support. The bolts must be long enough to provide for bolt extension beyond the fastening nut.

• Carriage bolts, or hex bolts with washers between the hex head and sign face, should be used to reduce the possibility that the sign might separate from the support upon impact. Flat washers and lock washers should be used at the nut end of the bolt.

• The bolts must be tightened sufficiently to prevent loosening, but not so tight as to distort the sign face.

LOCK WASHER HEX NUT

RECTANQULAR HEAD BOLT

Regardless of the type of support that is being used, there are three general rules that must be followed: (1) the top of the sign should be 9 ft (2750 mm) above ground level to reduce the possibility of intrusion into the passenger compartment upon impact, (2) the retaining bolts must be snug but not so tight as to distort the sign face, and (3) the bolts must be of the proper size and length to prevent the sign blank from separating from the support.

Fastening to a U-Channel. Signs are normally mounted on the front face of the U-channel. This is the widest face of the U-channel. Signs can also be mounted on the narrow face, as in back-to-back sign installations, for example, but the decreased surface contact with the sign can result in damage to the sign face due to environ­mental loads. Side mounting on a U-channel requires channel brackets, and brackets for mounting street name signs to the top of the post are available. Typical sign blank installations for a U-channel are presented in Fig. 7.38.

Fastening to Wood Supports. Sign installation on wood supports requires a bolt completely through the post and fastened with a nut. Lag bolts are not recommended for fastening sign blanks to the posts. Fastening with lag bolts is unpredictable upon impact because of the size of the bore hole, possible postsplitting, the presence of knots, and variable characteristics of the wood.

Fastening to Square Tubes. Sign installations on square tubes can be accomplished by %s-in (8-mm) bolts and nuts or by the use of rivets. Figure 7.39 presents typical sign panel fastening details for shaped wooden and square-tube sign supports.

Fastening to Round Steel Shapes. Signs should not be fastened directly to standard steel pipe or to light standards. The contact area between the back of the sign and the support due to the round support shape is too small to withstand the wind and other environmental loads. A number of different fastening methods can be used, including B-clamps, U-bolts, and stainless steel band clamps. Figure 7.40 presents the configura­tion of the B and stainless steel band clamps. The dimensions shown for the B-clamp are typical and vary slightly by manufacturer. The B-clamp is also available in other sizes than shown in Fig. 7.40.

FLAT WASHER

ND LOCK WASHER

50 MIN

FIGURE 7.38 Typical sign blank fastening to U-channel. Dimensions shown as mm: 50 mm = 1 in.

U-bolts can also be used to fasten signs to round supports. The U-bolt is attached to the sign by the use of Z-bar aluminum channel or pieces of U-channel. The U-bolt is purchased with an anchor chair to grip the post. Fastening details for U-bolts with aluminum Z-bar are presented in Fig. 7.41 and with aluminum channel or U-channel in Fig. 7.42.

Fastening to Structural Steel Shapes. Fastening signs to S – or W-shaped beam posts, often used with slip base designs, should be accomplished by using a stiffener. Clamps are used to fasten the stiffeners to the support, eliminating the need to drill into the support itself. Fastening details using aluminum Z-bar as the stiffener are presented in

Fig. 7.43. Figures 7.44 and 7.45 present fastening details for commercially available

aluminum stiffeners specifically designed for mounting signs to beam supports.

Slip base designs for single sign supports provide the opportunity to use stronger sign supports than would be possible without the slip base design. The purpose of the slip base is to provide a separation plane between the sign support and the anchor system. The two pieces are fastened together with bolts that must be properly tightened, or

torqued. If the bolts are not torqued enough, they will be loosened by vibration from environmental loads, causing the sign assembly to separate. If the bolts are torqued too much, the friction between the base of the signpost and the anchor piece will be too large to permit proper separation upon impact. A 20 to 28 gauge metal “keeper plate” should be inserted between the faces of the top and bottom slip bases to prevent the bolts from migrating out of the assembly (Art. 7.4).

There are three basic types of slip base designs for single sign supports. The hori­zontal slip base design (Fig. 7.25) will operate correctly when impacted from either the front or the back. Horizontal slip base designs do not provide the lift capability available from inclined or multidirectional designs. Horizontal slip bases when used as single sign supports therefore do not function as well upon impact as the other slip base designs.

The inclined slip base (Fig. 7.18) is the recommended type of slip base for single sign supports when impact can be expected from only one direction. Its performance upon impact is designed to cause the upper sign support and sign panel to raise up, thus allowing the vehicle to pass completely under the support assembly. The anchor piece of the inclined slip base must be installed so that approaching vehicles encounter the lower edge before the high edge.

The multidirectional slip base is fastened together with three bolts and has a lift cone fastened to the bottom plate. The sign support is tubular with a maximum size of 5 in (127 mm) diameter.

All of the slip base designs require a firm foundation for proper operation. Concrete foundations should be used for all slip bases, since direct burial may result

in base movement and improper release of the slip base. To prevent snagging of the vehicle undercarriage, no part of the anchor piece and its attached slip base may extend more than 4 in (100 mm) above ground level. Horizontal and inclined slip base designs can be constructed with wide-flange, standard-shape, and round sign supports. The concrete footing sizes for wide-flange and standard-shape signposts should be constructed to state specifications. The concrete foundation and anchor stub sizes listed in Table 7.11 are appropriate for round signposts with slip base designs.

Torque Requirements. Specifications for bolt tightness must be followed so that the sign assembly (1) remains intact under normal environmental loadings and (2) separates correctly upon impact. The specifications can be given in a number of ways, such as residual tension, clamping force, or torque. Torque refers to the amount of force used in tightening the nut to the bolt. The result of the nut-to-bolt tightening places the bolt in tension and exerts the clamping force. Measuring the torque is the most convenient method of obtaining a specified tightness. Providing a specified torque, however, does not guarantee a certain clamping force. Irregularities in the threads, heavy coating deposits on galvanized parts, or irregularities on the mating surfaces of the nut and plate faces result in friction forces. These friction forces cause an increase in torque to move the nut without a resulting increase in clamping pressure. To help ensure that the torque speci­fication provides the required clamping force, the nut should be tested for thread irregu­larities by being threaded on the bolt by hand. Also, the proper size flat washers should be used beneath the nut and the head of the bolt. The proper size bolts and recommended torque requirements are provided in Tables 7.12 and 7.13. The tables show requirements for round sign supports by diameter and for other support shapes by unit weight or mass. In U. S. Customary units, S6 X 12.5 designates a standard shape with a depth of 6 in and a unit weight of 12.5 lb/ft; in SI units, S150 X 18.6 member is a standard shape with a depth of 150 mm and a mass of 18.6 kg/m.

Slip Base Orientation. The proper operation of slip bases is also dependent upon proper assembly and correct orientation to the expected direction of impact. The parts,

and orientation to the primary direction of travel, for the three types of slip bases are presented in Figs. 7.19, 7.20, and 7.26. The horizontal slip base is generally not used for single sign supports. Where impact from more than one direction is expected, the multidirectional slip base provides better performance because of its design for lift upon impact.

Guidelines for Slip Base Installation. The following guidelines should be followed for slip base installation:

• Slip base installations require a firm foundation to operate properly upon impact. Slip base installations should always include a concrete base and never be directly buried or drilled and backfilled.

• Use proper size bolts for the slip base fastening. Bolts that are too small may not be able to be sufficiently tightened and may fail under environmental loads. Bolts that are too large may become stuck in the release slots upon impact and prevent proper separation.

• Use proper size and strength washers. The washers beneath the nut and head sur­faces should be sufficiently strong to withstand the torque requirements without deforming into the release slots of the base.

• Three washers should be used on each bolt—one each beneath the nut and bolt head, and one between the upper and lower faces of the slip base. The purpose of the washer between the two slip faces is to prevent binding upon impact. All galva­nizing runs or beads should be removed from both the upper and lower faces in the washer areas.

• The nut should be run by hand down the bolt to find thread irregularities that will provide inaccurate torque readings.

• Torque each base bolt to the required specifications.

• Remember that the top portion of the slip base must be attached to the anchor piece. Therefore it is recommended that the anchor piece be installed so that its highest portion extends no more than 3.5 in (90 mm) above the ground. This will help ensure that the addition of the top plate will not result in a height that can snag the undercarriage of an impacting vehicle. The installation shown in Fig. 7.37 is improper and can snag the undercarriage of an impacting vehicle.

• The bolts must be sufficiently long that they can extend approximately % in (10 mm) beyond the nut after complete assembly.

• Do not install an inclined slip base where impact from more than one direction is expected.

The correct installation of sign-support assemblies is dependent upon the type of sup­port post, the type of soil present, and the impact performance design of the sign assembly. Installation instructions are contained in standard state drawings and, for proprietary devices, from signpost manufacturers. Proper installation practices for the most common types of single support assemblies are presented in subsequent sections.

U-Channel Posts. The most common method of installing U-channel posts is by direct burial. The burial can be achieved by mechanical post drivers, by sledgehammer, or by digging a hole and backfilling. If the post is to be placed by driving into the ground, then a driving cap should be used to prevent damage to the end of the U-channel. Drive or place the posts at least 3 ft (900 mm) but no more than 3.5 ft (1100 mm) into the ground to make it easier to pull out damaged posts.

U-channel posts can also be installed as two-piece assemblies consisting of an anchor base and the post support. The advantage of the two-piece assembly is that the post will break off from the anchor piece upon impact. This often improves safety upon impact, makes repairs easier, and makes it possible to salvage portions of a damaged U-channel post. An anchor base assembly is especially advantageous when the post is placed in a paved area, such as a concrete median. The anchor piece should not extend more than 4 in (100 mm) above the ground to prevent snagging the vehicle undercarriage.

The anchor piece can be directly driven, buried 3 ft (900 mm) in the ground, or embedded 2 ft (610 mm) in a concrete foundation that is 8 in (200 mm) in diameter and 2.5 ft (760 mm) deep. The signpost can be attached to the anchor piece by a generic splice or the use of commercially available devices.

Figure 7.32 presents a generic method of attaching the signpost to the anchor piece. The signpost overlaps the anchor piece by 6 in (150 mm) to provide stability against the environmental loads. Since the anchor piece cannot extend more than 4 in (100 mm) above the ground, this means that the signpost is at least 2 in (50 mm) below ground level. The signpost is placed behind the anchor stub, and the posts are attached together

with two /-16-in (8-mm) bolts spaced 4 in (100 mm) apart. Extra %s-in (8-mm) nuts are used as spacers between the two post pieces to prevent binding during impact.

A number of commercial splicing devices for installing two-piece U-channel assem­blies are also available. Figure 7.33 provides installation information on the Eze-Erect system available from Franklin Steel, and Fig. 7.34 is information on the Minute-Man coupling from Marion Steel [35, 36].

The following guidelines should be followed for the installation and use of U-channel posts:

• If U-channel posts are driven into the ground, they should not be embedded more than 42 in (1100 mm), to make it easier to pull out damaged posts.

• Use a drive cap to drive the U-channel into the ground to prevent damage to the post end.

• If an anchor base is used, do not leave the anchor stub protruding more than 4 in (100 mm) above ground level.

• The generic splice should provide an overlap of 6 in (150 mm) with the anchor base. This results in 2 in (50 mm) of the signpost extending below ground level. The signpost is fastened to the anchor stub with /8-in (8-mm) grade 9 bolts spaced 4 in (100 mm) apart. The signpost and anchor piece should be separated with a %s-in (16-mm) spacer to prevent possible binding of the posts upon impact.

• Anchor pieces one size larger than the sign post will help prevent damage to the anchor piece upon impact.

• The Florida splice requires an overlap of 8 in (200 mm). This results in embedding of 4 in (100 mm) of the signpost below ground. The splice is secured with /8-in (10-mm) A307 bolts, 2 in (50 mm) long, spaced at 6 in (150 mm) center to center. A 58-in (16-mm) spacer is placed between the anchor piece and the signpost. The use of 58-in-diameter (10-mm) bolts requires that the post holes be reamed in order to insert the bolts. Reaming destroys the corrosion protection of the hole, necessitating the application of zinc-rich paint paste to prevent corrosion.

• If commercial splices are used, the manufacturer’s installation instructions must be closely followed for proper impact performance.

• The frangible bolt provided with the Minute-Man must be used for proper impact performance. Do not replace this bolt with a regular steel bolt.

• It is not recommended to interchange signposts and anchor stubs of different manufac­turers when there is variation in cross-section between the two sections. No crash tests have been done on mixed anchor stubs and signposts. The difference in cross-section may be sufficient to cause problems in nesting under some splice orientations.

• The signpost should be placed behind (on the nonimpact side of) the anchor stub for U-channel anchor base assemblies.

• Splices that are performed above the anchor piece to extend short pieces of U-channel or to piece together salvaged U-channel are not recommended. One-piece U-channel posts perform better under impact than posts that have been spliced above the anchor stub. A splice in the impact zone can strengthen the post and degrade its impact performance. Splices above the impact zone can open, allowing the sign panel to take an unpredictable and potentially hazardous trajectory. The splice can also open with the lower end of the upper post section penetrating the impacting vehicle. If splices above the anchor piece are used with U-channel, it is important that the following conditions are met [37]:

The splice does not extend below ground level.

The overlap is approximately 18 in (460 mm) fastened by four %s-in (8-mm) bolts, with two bolts, through the holes nearest the ends, at each end of the splice. Spacers 58 in (16 mm) thick should be placed over the bolts between the spliced pieces of U-channel.

A splice that is mostly below a vehicle bumper height should have a maximum top elevation of 20 in (500 mm), and a splice that is mostly above the bumper should have a bottom elevation of 16 in (400 mm) or above. A diagram of these recom­mendations is presented in Fig. 7.35.

Square Steel Tubes. Square steel tubes are available from a number of manufacturers in perforated, and punched but not perforated, styles [38, 39, 40]. Two of the major manufacturers of square-tube posts are Unistrut, with the brand name Telespar, and Allied Tube and Conduit, with the perforated Square Fit and the nonperforated Quick – Punch tubes. Square-tube sign supports can be installed as one-piece direct burial assemblies and with anchor pieces. The anchor piece assemblies have the advantages of more predictable performance upon impact, a larger range of permissible sizes, and reduced maintenance required for repair after impact. Figure 7.13 shows different installation methods.

Square steel-tube sign supports up to 2.25 in X 2.25 in (57 mm X 57 mm) in size can be installed by direct burial. Sizes larger than 2.25 in X 2.25 in (57 mm X 57 mm) require an anchor base assembly to provide acceptable impact performance character­istics. The most common method of direct burial is by driving directly into the ground, using a driving cap to protect the end, by mechanical drivers or a sledgehammer. Drive or place the square tube at least 36 in (900 mm) deep but no more than 42 in (1100 mm) into the ground to make it easier to pull out damaged posts.

Repair of damaged square tube is easier to perform when an anchor base assembly is used. The anchor base assembly for square tube usually consists of a 30-in-long (760-mm) anchor piece, one size larger than the signpost, and an 18-in-long (450-mm) stiffening sleeve, one size larger than the anchor piece. The sleeve provides a double­walled anchor base that helps prevent damage to the anchor assembly and makes the breakaway characteristics of the signpost more predictable. Acceptable impact perfor­mance can also be obtained by the use of only the anchor piece, but damage to the anchor piece and increased maintenance are more likely to occur than when using a stiffening sleeve. Sizes larger than 2.5 in X 2.5 in (64 mm X 64 mm) should not be used for breakaway performance with the anchor breakaway design. The anchor piece must not extend more than 4 in (100 mm) above ground level. The installation proce­dures for the square-tube anchor base system are provided in Fig. 7.36.

In addition to the telescoping anchor bases, made from larger sizes of square tubing, there are heavy-duty anchor bases commercially available. These bases can be used in hard or rocky soil conditions that can present problems for driving the regular-sized tubing as anchor pieces.

The following guidelines should be followed for the installation and use of square steel-tube signposts:

• Do not directly bury square steel tubing that is larger than 2.25 in X 2.25 in (57 mm X 57 mm). If a sign requires a larger post, use an anchor base system.

• Repair of the square steel-tube sign assembly is much easier if an anchor base system is used. The stiffening sleeve helps reduce damage to the anchor and provides a strengthened base for reliable impact performance.

• The anchor assembly should be driven or placed into the ground with only 1 to 2 in (25 to 50 mm) protruding above ground level. This will expose one or two holes for fastening the sign assembly, reduce vehicle sagging, and ease repair.

• If driving the post or anchor base into the ground, use a drive cap to protect the exposed end. If a drive cap is not used, the exposed end will become distorted, inhibiting insertion of the telescoping tube.

• Do not install a two-piece anchor assembly if the top of the anchor piece and sleeve is not flush or if the holes are misaligned. The bolts will be difficult to insert and the higher piece may bend upon impact, damaging the anchor assembly.

• Do not overtighten the bolts that fasten the signpost to the anchor assembly. Tightening the bolts too much will distort the tubing and hinder the removal or insertion of the signpost into the anchor assembly.

• Sections of square steel tube can be spliced together to allow the reuse of damaged posts. The splice is made by using a 12-in-long (300-mm) section of tubing one size smaller than the tubing to be repaired. The 12-in (300-mm) section is inserted halfway into one of the tubes and secured with two drive rivets or one bolt. The second tube is then slipped over the free end of the 12-in (300-mm) section and fastened in place.

• Square tube can be used to install signs in areas of concrete or asphalt by drilling or chipping through the surface and driving an anchor assembly in place. An anchor base is recommended in concrete or asphalt areas to make repair easier in case of impact.

Wooden Posts. The most common wooden supports for single signpost installation are the 4-in X 4-in (90-mm X 90-mm) shaped and the 4-in-diameter (100-mm) round posts. These posts should be directly buried to a depth of at least 36 in (910 mm) (Fig. 7.12a). Deeper burial is often performed to reduce vandalism. Posts larger than the 4-in X 4-in (90-mm X 90-mm) and the 4-in-diameter (100-mm) posts require drilled holes to reduce the cross-section and embedment in concrete so as to safely break away during impact. The requirements presented in Tables 7.8 and 7.9 should be followed for the installation of rectangular shaped and timber posts.

The use of!2-in-thick (13-mm) Styrofoam for the concrete foundation (Fig. 7.12b) eases the removal of broken stub pieces [41]. An example of hole placement to achieve a weakened cross-section is also presented in Fig. 7.12b [42]. The bottom hole should never be centered more than 4 in (100 mm) above the ground, because the stub piece must remain at 4 in (100 mm) or less after impact. Rectangular-shaped posts are placed with the long post dimension parallel to the direction of travel. The holes of the proper size for the post are drilled perpendicular to the expected direction of impact.

Steel-Pipe Posts. Steel-pipe (schedule 40) posts smaller than 2 in (50 mm) internal diameter can be directly buried and still provide acceptable impact performance. As

Post size, Embedment Comments and required

in (mm) type and depth post modifications

indicated in Art. 7.3.6, a plate 4 in X 12 in X 0.25 in (100 mm X 310 mm X 6 mm), or two sign clamps, should be bolted or welded to the pipe, beneath ground level, to prevent rotation due to wind. Schedule 40 steel-pipe supports should be direct buried, with the attached earth plate, to a depth of at least 42 in (1070 mm) to provide accept­able performance upon impact.

A breakaway collar assembly is required for schedule 40 standard pipe that is equal to or greater than 2 in (50 mm) ID. The breakaway collar can be made by the use of a regular pipe coupling or reducing coupling [43]. The reducing coupling is recom­mended since it reduces the probability of damage to the anchor piece, thereby easing repair. The anchor piece is usually one size larger than the signpost. The anchor assembly consists of a 24-in-long (610-mm) anchor piece placed in a concrete footing that is 30 in (760 mm) deep and 12 in (300 mm) in diameter.

In addition to standard steel pipe, there are round steel-tube sign supports available from a number of manufacturers, with a wall thickness of 12 gauge or less and designed for use in an anchor system. Commercial anchor systems, such as the Poz-Loc, can be used for the round steel tubes and for standard pipe 2 in (50 mm) or less in size [44]. The use of commercial anchor systems requires closely following the manufacturer’s instructions for proper performance.

A summary of steel-pipe sign-support installation recommendations is provided in Table 7.10. Also consider the following guidelines:

• Standard steel pipe (schedule 40) that is equal to or greater than 2 in (50 mm) ID must be of breakaway design with anchor base.

• Anchor pieces should be placed in a concrete foundation and be one size larger than the signpost. The breakaway mechanism can be achieved by the use of a reducing coupling. The top of the coupling should not be more than 4 in (100 mm) above ground level.

Also consider the following:

• Do not install aluminum round signposts larger than 3.5 in (90 mm) diameter. Recent tests show that the larger aluminum post sizes fail in weak soil conditions.

• Anchor plates or two sign clamps configured to encircle the post should be used below ground level to prevent rotation due to wind loads.

An important element of a safe highway environment is the proper construction and maintenance of traffic signs. Good designs and the best of materials will not be effective in reducing accident potential or severity if the traffic signs are improperly placed or installed. This requires that field crews be knowledgeable of proper installation tech­niques and that they report and correct any possible problems instead of merely placing the signs at the roadside. (See also Art. 7.1.2.)

7.6.1 Proper Placement

Important considerations for proper placement include the following:

• Warning signs should be placed sufficiently in advance that the driver has adequate time to perceive, identify, decide, and perform any necessary maneuver. A guide for the placement distance of warning signs is contained in MUTCD [2].

• The MUTCD provides guidelines on the height and lateral placement of typical sign installations. Signs installed on the side of the road in rural districts should be at least 5 ft (1500 mm) measured from the bottom of the sign to the near edge of pave­ment. In urban districts or where parking or pedestrian movements occur, the clear­ance to the bottom of the sign should be at least 7 ft (2100 mm). Ground-mounted signs on freeways and expressways should be at least 7 ft (2100 mm) above the level of the pavement edge and have the minimum lateral offset of 12 ft (3700 mm) from the edge of the traveled way. If a shoulder wider than 6 ft (1800 mm) exists, the minimum lateral offset for ground-mounted signs should be 6 ft (1800 mm) from the edge of the shoulder.

• All sign assemblies located within the traversable area must be capable of giving way safely upon impact. This requires that the maximum vehicle deceleration does not exceed 16 ft/s (5 m/s) and that the sign assembly does not protrude into the pas­senger compartment [14].

• Sign supports installed with anchor systems must have a maximum height of 4 in (100 mm) from ground level to the topmost part of the anchor.

• Most sign-support assemblies are designed to function properly when impacted at bumper height, typically about 20 in (500 mm) above the ground. If impacted at a higher point, the assembly may bind at the planned shear point resulting in nonacti­vation of the breakaway mechanism. For this reason, it is critical that breakaway sign assemblies not be located near ditches or on steep slopes or other locations where the vehicle can become partially airborne at the time of impact.

• Sign supports should not be placed in ditches. The water in the ditch can erode the soil around the base of the support, cause premature deterioration of the post, and freeze, resulting in unpredictable performance during impact. The ditch can also act as a guideway that directs errant vehicles into the sign assembly.

• Sign-support assemblies are tested in both strong and weak soils. Supports that are designed to yield, or fracture, upon impact generally perform better in strong soil. Strong soil holds the buried portion in position, providing sufficient resistance for the sign support to break near ground level. Weak soils do not provide this resistance, but permit movement within the ground and unpredictable results. Yielding or fracturing supports that are embedded less than 40 in (1000 mm) in weak soil will often pull out of the soil. While this may provide acceptable impact performance, the force of the wind and ice loads may cause the sign assembly to rotate or fall down. The actual soil type that is present may not be known until the start of installation. Weak soils are those that offer relatively little resistance to driving the signpost. If weak soils are encountered, there are measures that can be taken to maintain sign orientation in the face of environ­mental loads and still result in proper operation during impact. These include embedding the signpost to 40 in (1000 mm), and the use of anchor plates, concrete footings, and commercially available anchor systems [34].

• Single-sign-support systems are designed to operate safely when only one support is struck upon impact. Tests have shown that an errant vehicle, leaving the roadway at an angle, can impact more than one support if supports are not separated by more than 7 ft (2100 mm). This separation applies to other fixed objects as well as signposts. For example, a 3-in-diameter (75-mm) tree is sufficiently small to provide accept­able impact performance. Installing a sign support 6.5 ft (2000 mm) from this tree, however, can result in an errant vehicle’s impacting both the tree and the signpost. The combined effect of the tree and sign can provide unacceptable impact performance.

• Multiple-mount sign supports are required to support sign panels that are too large to withstand wind and ice loads with the use of only one support. Multiple-mount sign supports are designed to provide acceptable performance upon impact when the supports are placed 7 ft (2100 mm) or closer to each other. This close spacing results in the possibility that a vehicle leaving the roadway at an angle will impact two or more supports simultaneously. This possibility means that some supports approved for use in a single-support system are not approved for multimount designs. Support configurations that have not been approved for use as multiple – mount systems should not be used for multimount sign supports until they have been approved by the FHWA.

• Study the traffic patterns and surrounding geometrics prior to installing any sign. If the sign assembly can be expected to be struck from more than one direction, then a unidirectional slip base design is an improper choice. Two-lane rural roadways should use roadside supports that function safely when impacted from two direc­tions. Installations on freeways, with wide medians or positive median barriers, can be expected to be impacted from only one direction.

• In summary, be aware of what is required for sign installations to function properly for both the environmental loads and vehicle impact. Do not install a device simply because it has been specified on the sign order. The actual site conditions may have been unknown, or different from what was expected by the designer who specified the type of sign assembly. If problems are identified, contact a supervisor to deter­mine if changes should be made.

A number of manufacturers have developed products so that the anchor piece can be placed almost flush with the ground. These products can be used either to retrofit

existing slip base designs or for new installations. One such manufacturer is Transpo, which markets the Breaksafe breakaway system for ground-mounted signs. These devices use breakaway couplings and brackets designed for different support types and sizes. Included are back-to-back concrete and direct buried U-channel, 3- to 4%-in (75- to 114-mm) round pipe, 3- to 5-in (75- to 127-mm) square tube, and various sizes of wide-flange and standard beam shapes. The advantage of the retrofit is that proper torquing, to prevent blowdown or walking due to environmental loads, yet permitting slip during vehicle impact, is not required with the frangible coupling retrofit.